Amplification system for noncontacting thickness gauges



g- 1950 s. BERNSTEIN AMPLIFICATVION SYSTEM FOR NONCONTACTING THICKNESS GAUGES Filed March 10, 1949 Inve n or I Stanl ey Bern stein,

H i s Attorn ey.

F'atentecl Aug. 8 1 950 :AMZPLIEIGATION SYSTEM FOR NONQON- .T-AOTING THICKNESS GAUGES ".Stanley Bernstein, "Schenectady, N. Y., assignor to General Ele'etric Company, a. corporation of 1 New York AppBicatiomMarchJO, 1949,.:Serial*No.'80,71-6

,lMy invention relates to precision ithickness ogaugeso'f .the type employing radiations, suf'chas.

.sbetarrays, .emittedffrom a radioactive"sourceins .the measuring agent.

LlImgeneral, .al thickness gauge of the above type .comprehen'ds a source .of iisubatorfiic l'pa usu'eh as'beta rays, a suitable detector'ofithe'radiations emitted "from this. radioactive "sourcegan'd :means. to. measure the amount of. such radiations reachingithe'detector Whena materialfthe thickness vof whichlis to be. measured, is "interposed 'lbetween the source and'the .detector. In order xtog permit utilization of "fan alternating -"current amplification system 'and "to minimize ba'c'lb .ground" ra'diations; which mayicausespuriousi refsponsesiinithe aboveiitype detecton'it'fhasbeen suggested that a,"rotary"shutter. periodically "in- Lterrupt and "thereby modulate Tthe radiation *beamgsotthtt the" output of the detector'wilrhave or the "detector-until" the consequent" error voltage, "as amplified and indicated =on a suitable meter, is "zero. The amount-of*buckingvo1tage that isintroduced to obtain this null defiection point-is"caiibrated to indicate the =actua1 thickmess of the -materi'altwhich is measured. will he appreciated, however, that irr a systenr 'o this type the meter is use'd' oniy as a haIance indicatorgand that-amew s'etting. of the buckin'g 'voltv-age isrrequirit each time awmat'eriar of difierent thickness is measure'd. i-ilt will also be appreci- *atd that: amplification-terror isatminimizduoniy *atztheinull' ..deflectionrpoint and: that? the idefiectioniofzthexmeterithroughoutiits range (1088511013 accurately: represent the .deviation in 'ithickness s-of variousi 'specimenssor: portionsxof iai particular specimen.

A. further object otmy, invention;.theretorezzis togprovidea:measurement.circuits which; the zdeflections of; aaneter rrepre'sent withegreataaecuracy: the deviation in'the:thicknessxofmarious specimens .andccanflbe calibrated. to. read directly the thicknessthereof.

'JBecauSecof the...neces'sity-..of.multiplet.stages. amplification insuch thickness,gauges,..other cir- .c.uits, such v as conventional .reedback circuits, which. havei .been.-.suggested toireduce. amplification errors"..have. generallysacrificed thestability characteristics .ofltheramplifierrifajlarge amount of "feedbacklis use'fdlin order. to achieve greater accuracy. A further. specificiobj ect ofnnylinven .tion; therfore,11is' to provide: a new. and improved I feedback circuit, for. an. amplification system ploye'd 'inafthicknessrgauge of. the radiation-Z'b'earn modulated type -which 1 effectively ,rnininiizes amplification errors and yet"has good stability. Broadlystated, my. invention comprehendsn-an amplifier Tfor a radiation .beam modulated type of "non-(contacting thicknessjga'uge' in .which the amplified alternating component of thejoutp11t of an associatedrejdiationfietectoris converted into a'representative unidirectional voitagewhich is combined througha variable transcon'ductance amiolifierwithan alternating voltage of'the same 'frequency as that of'the modulated radiation fie 'tector' voltage. but "in phase fopposition" thereto. A traction of? this'cornbine'd' voltage" is; then tied 'ba'ck"'to theinput of the'amplifi'er inorderito compensateffor' any 'ehangein gain 'due to iamplification. "A"'meter, which "may be :calibratd 'to'read directly thethic kness of' a'mate'rial inter cepting the "radiatiombeam, "is preferably "connected in t the circuit-at aTpoint "of" lowinherent amplification" error arifimeasures: a".vo1ta'gej pro-- portional to the feedback "voltage. f-Means are also included in the circuit "whereby? the meter may-be adjusted. i' o=read*a change from"any"desired setting in order toypermitthe instrument to be used 'over'a 'i-imited rangeor, --converse 1y, to-extendthe useful'ra-nge ofmeasurement ofany particular instrument.

*aOtEIistic-ijf "my 'i-nvention' are=set forth--w'-ith particularity in theappendedcla-ims. l/ finitention itself, howeverg- 'oan besthe uriderstoodby ret erencezto the? toilou' i-ng description taken in connection -with' the accompanying drawing m-wmeh Fig. l 'is cir'cuit diagram of a thickness gauge embodying my inventionean'd- Fig. 2 is a -curVe explanatory of "the--operza;tionof the varia-ble .transcoridu'ctance amplifier included in the oircuit'offl ig -L' 5 Referring to Fig..-1, I haveeshown-=-rny -inven tion in 'one' tonm as including a radioactivesouree l.:of subatomic :zparti'cles, such: as beta-raysyenclosedawithinea;.shielciii2,vpreferablyklead,i whioh has; aifsma'l l awin'dowr monuoneasi'dei: through whioh radiations are emitted in the form of a beam 4. Although I prefer to use beta rays because their moderate range in the neighborhood of several feet of air or .100 inch of aluminum is particularly suitable for many industrial applications, many other types of subatomic particles such as alpha particles, protons, or neutrons, may alternatively be used, depending upon the thickness and composition of the material to be measured and the availability of a suitable radioactive source. Sources of beta particles are relatively easy to obtain. They are emitted, for example, from radium, strontium 90, carbon 14 and caesium 134.

A radiation detector sensitive to subatomic particles, such as an ionization chamber 5, is arranged to receive the beam of beta rays 4 emitted from source I. This ionization chamber 5 is also preferably enclosed within a shield 6. to minimize spurious electrostatic pick-up. A resistor 1 and a suitable high voltage source 8 are connected to form a series circuit'with the ionization chamber 5. As is well known in the art, the ionization chamber 5 functions as a variable impedance controlled by the number and rate of arrival of the ionizing particles and causes a voltage to appear across the resistor I which represents the intensity of the radiation beam 4. When a material 9 to be measured intercepts the beam 4, the beta rays are absorbed to a degree depending upon the thickness of the material 9; and the unabsorbed rays reaching detector 5 produce a voltage across resistor 1 which is a measure of the thickness of the material 9.

In order to produce an alternating signal voltage and to minimize the eiiect of spurious voltages which may appear across resistor 1 due to background radiations, a rotary shutter l driven by a motor I I is provided to interrupt periodically and thereby to modulate the beam of beta rays. The background radiations which produce these spurious voltages are principally of the type having great penetrating power and undergo relatively little attenuation due to the shutter I0. Background radiations of the less penetrating type are effectively absorbed by the shields 2 and 6. Therefore, the modulation frequency alternating component of the voltage across resistor accurately represents the thickness of an interposed material 9.

The system for minimizing the effects of background radiation and producing an alternating signal component described above is substantially identical to the system described and claimed in patent application, Serial Number 33,945, now Patent Number 2,488,269, filed by C. W. Clapp on June 19, 1948. As previously explained, my present invention is directed towards the provision of a convenient circuit by which the alternating voltage component generated by the radiation detector in such a system may be highly amplified and accurately measured. 1 The alternating signal voltage appearing across resistor I is first applied to an electrometer amplifier designated by block l2, which is preferably of the vacuum tube type, operated with a low plate voltage in order to reduce tube noise and grid current. This amplified voltage is applied to a voltage amplifier such as a conventional vacuum tube amplifier designated as block I3. The amplified voltage output appearing across a potentiometer i4 is then appliedto the controlling electrode of a vacuum tube l connected as a conventionalpower. amplifier'to produce an alternating current through an output transformer l6 responsive to the signal voltage. A voltage source H, a cathode resistor 18 and a bypass capacitor |9 provide the proper operating potentials for this power amplifier. It will be understood, of course, that although I have indicated only two stages of voltage amplification between the signal voltage and the power amplifier l5, additional stages may be included, if necessary.

In order to convert the alternating signal voltage developed across transformer l6 into a corresponding unidirectional voltage, I provide a phase sensitive detector, preferably in the form of a ring rectifier 20 in which one pair oi diagonally opposite corners are connected across the secondary of transformer l6 while the other pair of diagonally opposite corners are connected across the output winding of an alternator 2|. This alternator 2| is driven in synchronism with shutter It) by motor II, but produces an alternating output voltage which is adjusted to be out of phase with the voltage developed across resistor 1. Equal resistors 22 and 23 are connected as a voltage dividing circuit across the output winding of alternator 2| to permit a center tapped connection which may be grounded as indicated. A resistor 24 is connected between a center tap on the secondary of transformer 16 and ground. Since the frequency of the output of the alternator is equal to the signal frequency, the ring rectifier 20 functions in a. well known manner to cause a unidirectional current to flow through resistor 24, Whose magnitude and direction represent the amplitude and phase of the signal voltage. The rectifier elements 25 of the ring rectifier 20 are arranged to cause the unidirectional voltage developed across resistor 24 and applied to the grid of a variable transconductance amplifier tube 26 to become more positive as the amplitude of the alternating signal voltage increases and, conversely, to become less positive as the signal level decreases.

In order to provide a synchronized alternating current input for thevariable transconductance amplifier 26, the voltage developed by one half or the alternator output winding is connected through a series capacitor 21 and a voltage drop} ping resistor 28 across a cathode resistor 29. A load resistor 30 and a voltage source such as battery 3| complete the anode to cathode circuit of tube 26.

For the proper operation of this variable transconductance amplifier, it is necessary that the transconductance of the tube 26 vary together with its operating level. This may be easily accomplished by using a relatively high voltage source and an alternating current load whose magnitude is much smaller than the plate resistance of the tube. Typical values of this stageare 1000 ohms vfor-the alternating current load, 200 ohms for cathode resistor 29 and 300 volts for battery 3!. As indicated by the typical grid voltage-plate current curve of this variable transconductance amplifier illustrated in Fig. 2, the rate of change of plate current ip with respect to grid voltage 6,; at the lower end of the curve is much smaller than the rate of change at the upper end. Therefore, for a given magnitude of alternating reference voltage, er, applied between the grid and cathode of tube 26, the plate current and consequent amplification of the stage varies directly as the change in the direct current operating level of the tube. 'In the instant embodiment thls change in. operating level is dependent upon thechange in the imidircctional oltage dev lop d across res stor 24.

The output of th variable transconductance amplifier 2 6 is applied through a coupling condenser .32 .across a voltage dividing network such as resistors 33, and 34. Resistor 34 is preferably variable, as indicated. A fraction of this output voltage, depending .upon the relative magnitudes of resistors 33 and 34, is fed backto input of the electrometer amplifier l2 as illustrated. Since the alternating voltage applied to the cathode of tube 26 is l80 out of phase with the voltage developed across resistor l by the action of ionization chamber 5, the output voltage of the variable amplifier and consequently the fraction of this output voltage which is fed back to the input of amplifier I2, is also always 180 out of phase with the original signal voltage.

An alternating current voltmeter 35, which preferably is phase sensitive, is connected between the high voltage end of resistor 33 and the movable arm 33 of a potentiometer 31; which, in turn, is connected across one half of the output winding of the alternator Z I. Since the same half of the output winding of the alternator 2| is employed as the alternating reference voltage for tube 26, and since the voltage developed across resistors 33 and 34 is in phase with this reference voltage; the potentiometer 31 may be considered as being connected in parallel with the resistors 33 and 34. If the movable arm 36 of potentiometer 3'! is adjusted to its grounded end, the voltmeter 35 reads only the alternating voltage output of the variable transconductance amplifier as developed across resistors 33 and 34. By varying the position of arm 35 of potentiometer 31, a voltage point can be reached thereon which is equal in phase and magnitude to the voltage developed across resistors 33 and 34. Since the voltmeter 35 is connected between these two points an amount of alternating voltage can be introduced to permit the reading of voltmeter 35 to be zeroed at any particular thickness setting from which deviations are desired to be measured.

It will be appreciated that my invention may be aligned in difierent ways to fit various modes of operation. If it is desired to measure directly the thickness of materials with reference to zero thickness, the intercepting material 9 is removed, the arm 36 of potentiometer 31 is moved to its grounded position, and the value of resistor 34 is adjusted until the indicator of meter 35 is fully deflected. When a material 9 to be measured is thereafter interposed between the radioactive source I and the ionization chamber 5, the signal level is reduced, the direct current operating point of tube 26 is decreased and causes a corresponding decrease of the voltmeter reading. It is apparent that the meter 35 may then be calibrated to read thickness. directly.

If it is desired to measure the deviations in the thickness of an interposed material from a particular zero setting, it is only necessary to adjust the potentiometer 3'! until the desired level is reached.

Extreme accuracy of measurement can be obtained by employing sufiicient amplifier gain and by proper adjustment of the degeneration introduced by the feedback connection from the voltage dividing resistors 33, 34 to the electrometer amplifier I2. Since the degeneration encompasses the entire amplification system, and the meter 35 is connected to read this degenerated output, any change in amplification is minimized in direct proportion to th amount of degenerae tion introduced. Therefore, for optimum results, the magnitudes of resistors 33 and 34 are chosen so that a very large percentage of the signal voltage developed across resistor l is desenermv tively fed back from the output of the variable transconductance amplifier.

Because of the introduction of a reference voltage in the variable transconductance amplifier which is in frequency synchronism with the signal voltage. and because of the conversion of the alternating signal voltage into a unidirectional biasing voltage for the variable amplifier, it is apparent that any phase shift oi. thesignal voltage which may result from many stages of amplification does not affect the feedback signal; Since the feedback voltage is always out of phase with the signal voltage, there is no insta-. bility due to possible phase reversal as .in the conventional multistage amplification systems, and consequently the amplification may e increased to. any desired extent.

It is to be understood that although I have shown. a particular embodiment of my invention, I do not wish to be limited thereto, since many modifications may be made; and I intend by the appended claims to cover all such modi fications as fall within the true spirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1., In a thickness auge employing as its measuring agent a constant beam of subatomic particles impinging upon a detector thereof and having means to modulate periodically the particle density of said beam to produce an alternating signal voltage output from said detector, alter nating current amplification means to amplify said alternating signal voltage, means to convert said amplified signal voltage into a representative unidirectional voltage, means synchronized with said modulating means to provide a substantially constant alternating voltage 180 out-of-phase with said output voltage of said detector, means to vary the magnitude of said outof-phase voltage responsive to variations in said unidirectional voltage, a fraction of said out-ofphase voltage of varying magnitude being supplied back to the input of said alternating current amplification means, and means to indicate the magnitude of said varying out-of-phase voltage.

2. In a thickness gauge employing as its measuring agent a constant beam of subatomic particles impinging upon a suitable detector thereof and having means to modulate periodically the particle density of said beam to produce an alternating signal voltage from said detector, means including an alternating current amplification system to amplify said signal voltage, means synchronized to said modulating means to provide an alternating voltage of substantially constant amplitude 180 out-of-phase with said signal voltage, said out-of-phase voltage being connected to and cooperating with a phase sensitive detector to convert said amplified signal voltage into a representative unidirectional voltage, means including a variable transconductance amplifier to vary the magnitude of said out-of-phase voltage responsive to variations in said uni-directional voltage, a fraction of said varying out-of-phase voltage being degeneratively fed back to the input of said alternating current amplification system, and means including a meter connected to the output of said variableamplifier to indicate a voltage' proportional to said feedback voltage. I

' 3. In a thickness gauge employing as its measuring agent a constant beam of subatomic particles impinging upon a suitable detector thereof and having a rotating shutter arranged to interrupt and to modulate periodically the particle density of said beam to producean alternating signal voltage from said detector, alternating current amplification means to amplify'said signal voltage, means to minimize errors of measurement introduced by said amplification means comprising an alternator synchronized to said shutter and producing a substantially constant alternating voltage 180 out-of-phase with said detector output voltage, a phase sensitive detector connected to receive said amplified signal voltage and cooperating with said alternator out-' put voltage to produce a uni-directional voltage proportional to the magnitude of said amplified signal voltage, and a variable transconductance amplifier having its input circuit connected both to said unidirectional voltage and said alternator output voltage to produce an alternating voltage 180 out-of-phase with said detector output voltage which varies in accordance with said unidirectional voltage, said alternating current amplification means having its input connected to receive the algebraic sum of said detector output voltage and a fraction of said varying out-ofphase voltage.

4. In a thickness gauge employing as its measuring agent a constant beam of subatomic particles impinging upon a suitable detector thereofand having a rotating shutter arranged fo iiiterrupt and to modulate periodically thepartlcle density of said beam to produce an alternating signal voltage from said detector, an alternating current amplification system to amplify said sig nal voltage, means to minimize errors of measurement due to variations in the gain of said amplification system comprising an alternator synchronized to saidshutter and producing an alternating voltage 180 out-of-phase with said detector output voltage, a phase sensitive detector connected to receive said amplified signal voltage and cooperating with said alternator output voltage to produce a unidirectional voltage proportional to the magnitudeof said amplified signal voltage, a variable transconductance am plifier having its input circuit connected both to said unidirectional voltage and said alternator output voltage to produce in its output circuit an alternating voltage 180 out-of-phase' with said detector output'voltage and which varies in accordance with said unidirectional voltage, a determinable fraction of said varying out-ofphase voltage being supplied back to the input of said alternating current amplification system to degenerate the output thereof, and analternating current voltmeter connected across the output of said variable transconductance amplifier to indicate a voltage proportional to said degenerated output voltage.

' STANLEY BERNSTEIN.

No references cited. 

